7.1.3: Locomotion

Getting Around: Primate Locomotion

One of the things that makes primates fascinating is how many different ways they get around. Most animals are specialists, built for one primary mode of movement. Primates, by contrast, are locomotor generalists, and across the Order you find animals that climb, leap, swing, scamper, walk, and hang, sometimes all in the space of the same afternoon. The four main locomotor patterns seen in living primates are vertical clinging and leaping, quadrupedalism, brachiation, and bipedalism, and each comes with its own set of anatomical specializations.

Vertical clinging and leaping is exactly what it sounds like. The animal clings upright to a vertical branch, pushes off with its powerful hind legs, and launches itself to another vertical support, landing feet-first. Animals that move this way tend to have longer legs than arms, long fingers and toes for gripping, and notably elongated ankle bones that function like a springboard, giving extra leverage on the push-off.

Quadrupedalism, moving on all fours, is the most common locomotor pattern among primates, and you probably picture it anytime you think of a monkey walking along a branch or a baboon crossing an open field. The details vary depending on where a species spends most of its time. Arboreal quadrupeds tend to have shorter limbs and longer tails, which help lower their center of gravity and maintain balance on narrow, unstable branches. Terrestrial quadrupeds have longer limbs for covering ground efficiently and often shorter tails, since balance on flat terrain is less of a challenge.

Brachiation is the one you have personal experience with, if you ever played on “monkey bars” as a kid (monkeys can’t actually brachiate, so really these would be more appropriately called “ape bars”). Brachiators swing below branches hand over hand, using their arms like pendulums. It is an efficient and fast way to move through a forest canopy, but it requires a very specific body plan: arms longer than legs, highly mobile shoulders and wrists, a short and stiff lower back, and no tail. These are precisely the traits you see in apes, and we will return to them in the next chapter.

Some primates practice semi-brachiation, a hybrid strategy. They swing below branches like brachiators but also walk on all fours when on the ground, so their arms and legs are more similar in length. What sets semi-brachiators apart is their prehensile tail, a fully grasping tail that functions as a fifth limb. The underside of the tail even has a tactile pad, similar to the fingerprint ridges on your hands, for a better grip. This is a feature found only in some New World monkeys, making it one of the more distinctive traits in the primate world.

Finally, there is bipedalism: walking upright on two legs with a striding gait. Other primates will occasionally stand and walk on two feet, but they do so awkwardly and only for short distances. Humans are the only mammals who have evolved full-time striding bipedalism as a primary way of getting around, and it required a sweeping set of anatomical changes to the skeleton that we will explore in depth in later chapters.